1. Field of the Invention
The present invention relates to optical systems. More specifically, the present invention relates to mechanical systems and apparatus used to stabilize an optical line-of-sight.
While the present invention is described herein with reference to an illustrative embodiment in a particular application, it is understood that the invention is not limited thereto. Those of ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications and embodiments within the scope thereof.
2. Description of the Related Art
The current state of the art is such that guided missiles may be equipped with an onboard tracking and guidance system that provides a `fire and forget` capability. Such capability is desirable in that it permits a launch platform to either launch the missile and move to another tactical position or acquire a second target.
Missiles with an onboard tracking and guidance capability can also effectively meet mission objectives when merely launched into an area within which the target is expected to enter or exist. In this application, the missile acquires and tracks the target while guiding itself to the area or point of contact. This allows for an accurate system which permits a number of missiles to be launched at targets which would not otherwise be within range.
There are numerous missile guidance technologies which provide some or all of these capabilities. Of these, radar guided and optically guided systems are most commonly used. For the purpose of this application, optically guided systems are deemed to include active and passive systems using infrared, laser, and visual targeting and guidance techniques.
The accuracy of most, if not all, of the optical systems is dependent in some measure on the extent to which the line-of-sight on the target is maintained stable on a photodetector or sensor. That is, the incoming optical beam or image must be maintained on the sensor with good acuity and resolution, viz., minimal distortion and/or smear.
This is problematic inasmuch as a missile in flight typically experiences substantial disturbance forces. These forces may be due to atmospheric conditions or torques generated by the onboard guidance system. In any event, the forces tend to jitter components in the optical train causing distortion, smear, image offsets, and other problems.
To stabilize the optical train of missiles in flight, one prior approach has been to mount the sensor on gimbals in the line-of-sight of the input image. However, due to the mass properties of the sensor, this direct approach has been found to be limited. That is, the weight and size of the sensor make it difficult to stabilize the gimbals within the limited space available in a missile nose cone. The additional mass associated with any nuclear shielding would further exacerbate the problem. Thus, a more common solution has been to place a mirror on a gimbal in the optical line-of-sight which deflects the input image to an off-axis sensor.
The mirror is mounted on the inner gimbal and initially stabilized with respect to inertial space. The mirror deflects the input image to an off axis sensor. However, because of the 2 to 1 angular relationship between perturbations in the line-of-sight and perturbations in the reflected image, merely stabilizing the mirror with respect to inertial space is not enough.
To compensate for the mirror effect, many systems endeavor to measure a number of parameters including the missile rotation rate, the instantaneous gimbal angle relative to the sensor and the gimbal angle rotation rate relative to the sensor. These measurements are then used to calculate the mirror correction.
This approach thus provides indirect stabilization in that the correction is provided by calculation, not by a mechanical stabilization on the gimbal per se. However, such indirect stabilization systems cannot always adequately decouple the line-of-sight. That is, the mirror is not stabilized relative to the input image, and only indirectly with respect to an inertial frame of reference. As such, the accuracy of such systems is limited by the performance of the overall system. Consequently, such systems frequently have insufficient servo bandwidth to achieve a high degree of line-of-sight stability. That is, the accuracy of the overall system is dependent on the accuracy of the least accurate component on the measurement and computational chain. In addition, hardware for making the measurements and corrective calculations adds to system weight, cost, complexity and failure risk. Thus there is a general need for a simple, low cost mirror stabilization scheme which offers improved stability relative to those illustrated by the related art.